1. Statement of the Technical Field
The invention concerns communications. More particularly, the invention concerns an application of an adaptive chaotic spread spectrum systems and methods to adaptive link communications.
2. Description of the Related Art
There is an ongoing need to provide communications links that are optimized for communication devices located at various distances from one another. For example, direct communication links can be implemented using microwave, UHF, or VHF frequencies for line-of-sight communications. Where no line-of-sight exists between two communication devices, communications links can be implemented using earth orbiting satellites to relay communications signals. However, it is well known that implementing and maintaining wireless communications links with systems with fixed operational parameters are inefficient and can cause and be susceptible to various types of interference. Moreover, such links tend to be bandwidth limited so that data throughput is often greatly restricted. Communication systems with adaptive link parameters are known to those skilled in the art. However, certain chaotic waveforms and hybrids thereof have characteristics which provide superior capabilities in certain adaptive link applications. One such adaptive link application is non line-of-sight communication requiring assured secure communication systems, wherein levels of security and the ability to preserve the link can be traded in real time. Non line-of-sight channels often rely on multipath characteristics to enable their ability to provide such capability. However, by their very nature these channels are severe and require robust waveforms and processing. HF is an example of just such a channel.
Spread spectrum systems have the potential to provide robust communications in the severe time-varying bands. A pseudorandom number generator (PRNG) can be used as a means to generate spreading codes for such systems. A PRNG for such systems must generally utilize digital logic or a digital computer and one or more algorithms to generate a sequence of numbers. However, while the output of conventional PRNG may approximate some of the properties of random numbers, they are not truly random. For example, the output of many PRNG have cyclostationary features that can be identified by analytical processes. This is not desirable in a communication link that is intended to be secure and robust.
Chaotic systems can generally be thought of as systems which vary unpredictably unless all of its properties are known. When measured or observed, chaotic systems do not reveal any discernible regularity or order. Chaotic systems are distinguished by a sensitive dependence on a set of initial conditions and by having an evolution through time and space that appears to be quite random. However, despite its “random” appearance, chaos is a deterministic evolution.
Practically speaking, chaotic signals are extracted from chaotic systems and have random-like, non-periodic properties that are generated deterministically. In general, a chaotic sequence is one in which the sequence is empirically indistinguishable from true randomness absent some knowledge regarding the algorithm which is generating the chaos.
Some have proposed the use of multiple pseudo-random number generators to generate a digital chaotic-like sequence. However, such systems only produce more complex pseudo-random number sequences that possess all pseudo-random artifacts and no chaotic properties. While certain polynomials can generate chaotic behavior, it is commonly held that arithmetic required to generate chaotic number sequences requires an impractical implementation due to the precisions required.
Communications systems utilizing chaotic sequences offer promise for being the basis of a next generation of low probability of intercept (LPI) waveforms, low probability of detection (LPD) waveforms, and secure waveforms. While many such communications systems have been developed for generating chaotically modulated waveforms, such communications systems suffer from low throughput. The term “throughput”, as used herein, refers to the amount of data transmitted over a data link during a specific amount of time. This throughput limitation stems from the fact that a chaotic signal is produced by means of an analog chaotic circuit subject to drift.
The throughput limitation with chaos based communication systems can be traced to the way in which chaotic circuits have been implemented. The reason for reliance on analog circuits for this task has been the widely held conventional belief that efficient digital generation of chaos is impossible. Notwithstanding the apparent necessity of using analog type chaos generators, that approach has not been without problems. For example, analog chaos generator circuits are known to drift over time. The term “drift”, as used herein, refers to a slow long term variation in one or more parameters of a circuit. The problem with such analog circuits is that the inherent drift forces the requirement that state information must be constantly transferred over a communication channel to keep a transmitter and receiver synchronized.
The transmitter and receiver in coherent chaos based communication systems are synchronized by exchanging state information over a data link. Such a synchronization process offers diminishing return because state information must be exchanged more often between the transmitter and the receiver to obtain a high data rate. This high data rate results in a faster relative drift. In effect, state information must be exchanged at an increased rate between the transmitter and receiver to counteract the faster relative drift. Although some analog chaotic communications systems employ a relatively efficient synchronization process, these chaotic communications systems still suffer from low throughput.
The alternative to date has been to implement non-coherent chaotic waveforms. However, non-coherent waveform based communication systems suffer from reduced throughput and error rate performance. In this context, the phrase “non-coherent waveform” means that the receiver is not required to reproduce a synchronized copy of the chaotic signals that have been generated in the transmitter. The phrase “communications using a coherent waveform” means that the receiver is required to reproduce a synchronized copy of the chaotic signals that have been generated in the transmitter.
Chaotic waveforms have differing characteristics which are dependent on how the chaos is generated and its target application. For example, chaos can be generated with a Gaussian distribution for use in maximum entropy communication systems with maximum channel capacity or low probability of intercept and low probability detection. Gaussian distributed chaos can be used as a spreading sequence in a chaotic spread spectrum communication system. One practical downside to the use of Gaussian distributed spread spectrum chaotic communications waveforms is the peak-to-average power ratio (PAPR). Typically, the PAPR in a chaotic spread waveform is about thirteen decibels (13 dB). This means that the instantaneous peak power level of the chaotic spread waveform signal is thirteen decibels (13 dB) or twenty (20) times greater as compared to the average power level. In order to avoid significant distortion, any high power amplifier (HPA) used with such a waveform is operated at a gain level such that the instantaneous peak power levels do not result in overdriving the amplifier. However, if the PAPR is thirteen decibels (13 dB), this means that, on average, the amplifier output power is thirteen decibels (13 dB) lower than the maximum or peak power output that the HPA is capable of providing. This is sometimes referred to as HPA back-off.
For LPI/LPD applications, the reduced amplifier gain necessitated by a 13 dB PAPR is not a major concern since the goal is to reduce transmitted power as far as possible, and transmitters in such instances are not usually operated close to compression points. Other operational scenarios however, like satellite communications waveforms, power challenged systems, and anti-jamming waveforms to name a few benefit from the ability to emit as much power as possible without signal distortion to mitigate natural or hostile interference. Reducing the HPA back-off permits a higher transmitted power and therefore a direct contribution to link margin, which provides improved link communications capabilities. For example, increased signal-to-noise ratio at the receiver. Thus there is a need for chaotic waveforms that retain many of the advantages of coherent chaotic communication systems and provide a low PAPR.